Injection molding machine and injection molding method for manufacturing foamed shaped parts

ABSTRACT

The invention relates to an injection molding machine for the injection molding of foamed shaped parts made of polymeric materials in an injection molding method by introducing a physical expanding agent into the melt flow. The injection molding machine comprise a screw plunger ( 3 ), which rotates inside an injection unit ( 2 ). According to the invention, the screw plunger ( 3 ) has, downstream from a metering zone a porous or permeable section ( 16 ) which can be charged with the expanding agent via an expanding agent feed device ( 18 ) in the screw shaft of the screw plunger ( 3 ) and which two-dimensionally introduces the expanding agent into the melt.

[0001] The invention concerns an injection molding machine and an injection molding method using a physical blowing agent for the injection-molding of foamed parts.

[0002] The injection moulding methods used to manufacture moulded parts include not only compact injection moulding, but also foamed injection moulding and other special methods. Structural foam moulded parts have in contrast to compact components a sandwich structure, i.e. a more or less compact skin and a closed-cell core. They are characterized by good material properties and, in addition, are of economic interest. For example, their geometrical moment of inertia is displaced to the surface layer, thus giving them a higher specific rigidity than compact parts. Moreover, owing to their low warpage, reduced internal stresses, and few sink marks, ribbed moulded parts with sudden transitions in wall thickness can be manufactured with a high degree of dimensional stability virtually without problems. And because foaming gives rise to an internal melting pressure, dwell pressure no longer needs to be applied, with the result that large-area moulded parts can be produced with low locking pressures on smaller machines. The reduction in density leads not only to savings in raw material costs, but also to a reduced component weight. Sound and heat insulation as well as improved resistance to the manufacturing media round off the properties of foamed components.

[0003] A thermoplastic foam is generated with the aid of blowing agents which can be dosed in various ways to the polymer melt so that they mix to form a single-phase solution. Analogously to the conventional injection molding method, using standard injection molding technology, the quantity of material needed to charge the mould chamber is melted in the plasticizing cylinder. The plasticizing cylinder of a standard injection molding machine is characterized by a cylinder with nozzle and a plasticizing screw with a back flow stop. During the so called plasticizing phase, plastic granulate is processed from the feed hopper over the back flow stop to the nozzle by rotating the plasticizing screw. The plastics material is melted by heat transmitted from the walls of the plasticizing cylinder. While rotating, the plasticizing screw moves in axial direction against a defined ram pressure in the direction of the feed hopper and releases thereby volume in the screw's antechamber. After having completed the dosing phase, the material dosed in the screw's antechamber is injected in the most cases with high speed into the cavity of the tool by the axial movement of the plasticizing screw. Due to the pressure drop during the flow of the melt into the cavity, bubbles are generated by the expansion of the blowing or expanding agent. Thereby a foamed structure is developed. The foam structure is fixed by cooling or heating steps depending on the type of material used, such as elastomeric or duroplastic materials. The obtainable foam densities and the plant technology needed for the manufacture depend on the type and quantity of the blowing agent used.

[0004] There are basically two types of blowing agent, chemical and physical, whereby the difference between the two lies more in the type of dosing than in the initiation of foaming.

[0005] Chemical blowing agents are mixed in the solid state with polymer granulate and decompose under the action of heat, releasing one or more fluids in the process, in most cases nitrogen, carbon dioxide, or water. The drawbacks are the other products of decomposition that can lead to degradation of the polymer matrix, a falling off of mechanical properties, discoloration in the component, and corrosion and soiling of the mould. Furthermore, the relatively low gas output from the decomposition of chemical blowing agents achieves only limited degrees of foaming.

[0006] Fluids that are dosed directly into the polymer melt are called physical blowing agents. These can be inert gases such as nitrogen or carbon dioxide, also hydrocarbons such as pentane, as well as water. Physical blowing agents can obtain considerably higher degrees of foaming. And because there are no decomposition products, there can be no discoloration, and no detriment to mechanical properties. The drawbacks always listed in the past were the complex plant technology and the difficulty in controlling the dosed quantities owing to the unsteady state of the injection molding process.

[0007] Before a thermoplastic foam can be generated in the injection molding process, a polymer/blowing agent solution must be generated under high pressure.

[0008] Here, the expanding fluid is brought into contact with the low-viscosity polymer. Depending on the process conditions, diffusion processes then take place, leading to the absorption of the blowing agent in the melt. After enough time has passed, a single-phase polymer/blowing agent solution has formed.

[0009] The methods used today differ greatly in the way they bring the expanding fluid into contact with the melt.

[0010] One way of achieving a highly uniform solution of blowing agent in the polymer is to charge the material with expanding fluid beforehand. Here, a high-pressure gasification system is used to charge the polymer with carbon dioxide before the polymer is processed. The plastic granulate is pressurized to a predefined value with CO₂ in an autoclave at room temperature, whereby the polymer absorbs gas owing to the difference in concentration and pressure. The concentration of gas in the polymer is a function of the gasification time, amongst other factors. Once the saturation concentration has been reached, the pressure is reduced to the ambient value, and the gas-charged polymer is dosed to the injection molding machine via the feed hopper. The material is then melted and homogenized in the plasticizing cylinder, whereby the rise in pressure causes the dissolved gas to lie along the cylinder. On leaving the nozzle, the polymer foams as a result of the rapid drop in pressure.

[0011] The drawbacks with this method of precharging in autoclaves lie in the batch charging of the granulate (making it inappropriate for industrial applications) and the lack of flexibility with respect to time (the blowing agent diffuses continuously out of the polymer). So this method does not find practical application in industry.

[0012] Another method makes it possible to dose the blowing agent directly in the screw's antechamber. Underlying this method is a special gas injection nozzle flanged between the cylinder and the mould injection nozzle (DE 198 53 021 A1). The heart of this gas injection nozzle is formed by an annulus of porous or gas permeable sintered metal through which the melt passes during the injection process. A torpedo centered in the melt channel divides the melt upstream of the annulus for the best fluid engineering properties and recombines the two flows without dead corners once they have passed the annulus. If necessary, static mixer and shear elements can be installed for the homogeneous distribution of the polymer/blowing agent solution. The gas is fed by a gas dosing station that can regulate the mass flow to vary the proportion of blowing agent in the melt, thus obtaining various degrees of foaming.

[0013] Another technology is based on the injection of a physical blowing agent into the plasticizing cylinder of an injection molding machine (EP 0 952 908 A2). Here, the blowing agent is injected through several axially arranged radial apertures in the plasticizing cylinder's melt chamber. Upstream of each of these apertures is a controllable valve that can open and close the connection to the blowing agent supply. A cascade controller then correlates the controlled valve states with the position of the screw during the dosing process, i.e. the valves are opened and closed in succession. The purpose is to obtain the most uniform injection of the blowing agent as possible into the melt. Long mixing zones then homogenize the blowing agent/polymer solution, which in the ideal case is ready for the mould injection process as a single-phase substance.

[0014] Another process for the foaming of plastic parts implies the injection of a blowing agent in the area of an extruder (EP 1 072 375 A2). The blowing agent is added to the molten polymer through a porous region in the screw of the extruder. This process is used predominantly for the continuous production of plastic profiles by an extrusion process. To allow for injection molding type applications, such an extruder has to be combined with a plunger injection unit. The blowing agent is added in the extruder. In order to adapt this solution to a batch process, requires increased machinery and plant equipment alongside with the need for a special machine adapted specifically to the requirements.

[0015] The drawbacks of this method with the gas injection nozzle lie in the homogenization of the melt. During the mould injection process, there is, of course, only this injection time available to charge the melt with the blowing agent. And because foamed injection molding applications generally require a high mould injection speed in order to generate a high nucleation density, these times may not be long enough—despite the short diffusion paths in the annulus—to obtain an adequate and homogeneous sorption. On the other hand, feeding the blowing agent during the dosing phase is also problematic because the volume of the melt in the annulus is given by the geometry. So if the volume of the injection molded part is larger than the volume of the annulus, then this difference in volume is not charged with the blowing agent, and the molded part's foam structure will become inhomogeneous. If, on the other hand, the volume is smaller, then a part of the mass will be charged during two successive dosing phases. This is not associated with any problems as long as a single dosing cycle is adequate to enrich the melt with the blowing agent in the saturation state. In addition, static mixing elements involve the risk of damage to the polymer matrix caused by friction heat when high shearing forces act on thermally sensitive materials. Owing to the high pressure during the mould injection process a dosing station is needed to compress the expanding fluid to the appropriate high value.

[0016] The main drawback of this variant method injecting gas into the plasticizing cylinder is that a special injection molding machine becomes necessary. The cascade control system requires a correlation interface with the machine controller, as is not the case with customary machines. Because the melt is enriched with the blowing agent during the dosing phase, in the course of which the screw completes its axial travel back, several gas injection ports with the correspondingly complex valve technology are needed to obtain an approximately uniform dosing of the blowing agent in the melt. This complex plant technology does not only require high investment costs; it is a well-known fact that the more complex the applied technology becomes, the greater its susceptibility to malfunctions in the production run, and the higher the servicing costs. Guaranteeing that the blowing agent can disperse adequately in the melt after the point injection requires unusual mixer zones, which in turn require a special screw with long mixing elements and, as a consequence of the longer screw lengths, a special drive unit as well. Also needed is a dosing unit to inject the expanding fluid.

[0017] The invention is based on the problem of how to obtain high reproducibility and process reliability in introducing and distributing a physical blowing agent uniformly into the melt flow of an injection molding machine so that a homogeneous polymer/blowing agent solution is the result—and this with a conventional injection molding machine.

[0018] The invention resolves this problem on the strength of the features described in Claims 1 and 15. The advantageous feature of this invention is that the plasticizing screw upstream of a metering zone has a porous or permeable section which is pressurized with the blowing agent supplied by an blowing agent feed device in the shaft of the plasticizing screw and which introduces the blowing agent through the whole surface of the melt.

[0019] This invention makes it possible to use a conventional injection molding machine, after minor modifications, to manufacture physically expanded structural foam molded parts that are characterized by a compact skin and a foamed core and so combine improved material properties with savings in weight, material, and therefore costs when compared with compact components. Furthermore, no intervention is needed in the machine controller, so the investment costs are low.

[0020] Compared with the state of the art, the invention offers the following advantages:

[0021] low investment costs because no cost-intensive special machine is needed, but solely the replacement of the plasticizing screw of a conventional injection molding machine,

[0022] a uniform introduction of blowing agent owing to a flat, rotating gas injection zone that also travels during the dosing phase of polymer,

[0023] a high degree of homogenization owing to intensive shearing and mixing processes, whereby the effective length of the screw's mixing and shearing zones remains unchanged in the course of the gas injection,

[0024] the optimal solution behavior owing to long diffusion times and large diffusion areas with short diffusion paths,

[0025] process reproducibility independently of the dosing volume,

[0026] a high blowing agent efficiency.

[0027] Because the blowing agent in the porous or permeable section is introduced through the whole surface of the polymer melt, a uniform introduction of blowing agent is possible when the polymer is being dosed. The result is an improved solution behavior owing to long diffusion times and large diffusion areas with short diffusion paths. In addition, both a high reproducibility of the injection molding process independently of the dosing volume and the optimal utilization of the blowing agent can be established. Finally, the invention has the advantage of low investment costs because no cost-intensive special machine is needed, but solely the replacement of the plasticizing screw in the conventional injection molding machine. An extended plasticizing cylinder is not needed either. A length for the plasticizing cylinder of about 20 to 25 times of the plasticizing screw's external diameter is enough.

[0028] One advantageous feature is that the diameter of the plasticizing screw is smaller in the area of its porous or permeable section. The resulting greater depth of the screw base makes it possible owing to the polymer melt's low pressure level in the gas injection area to feed the blowing agent directly and without the need for a dosing station.

[0029] One advantageous feature is that the blowing agent is fed to the plasticizing screw during the dosing phase via a high-pressure sealing case enclosing radially the plasticizing screw. This physical blowing agent is provided as a fluid.

[0030] The high-pressure sealing case receives the blowing agent from at least one pressure cylinder. This has the advantage that no dosing station is needed.

[0031] The high-pressure sealing case traverses, without rotating, in the axial direction simultaneously with the axial movement of the plasticizing screw. This makes a uniform introduction of blowing agent possible owing to the flat, rotating gas injection area that also moves in the axial direction when the polymer is dosed.

[0032] The polymer/blowing agent solution is homogenized by the plasticizing screw's mixing and shearing elements whose effective lengths remain unchanged during the course of gas injection. The blowing agent is injected during the dosing phase.

[0033] The following gives a more detailed description of the invention with reference to the only drawing.

[0034] The only FIGURE shows a plasticizing screw (3) of an injection molding machine (1) with a plasticizing screw (3) rotating in the plasticizing cylinder (2).

[0035] A feed hopper (5) introduces the polymer granulate to the plasticizing screw (3), from which the granulate is drawn into the feed zone (6). External cylinder heating (12) assists the adjoining compression zone (8) and metering zone (10) to melt, compress, and homogenize the polymeric material so that a polymer melt is available at the end of the metering zone (10) with thermal and material homogenization. At the end (14) of the plasticizing screw's (3) metering zone (10), the screw base is abruptly increased, i.e. the plasticizing screw's (3) diameter is abruptly reduced. Installed in this area of reduced plasticizing screw (3) diameter is a porous or permeable section (16) that is pressurized with a physical blowing agent supplied by an blowing agent feed device (18) through a bore (20). The blowing agent is introduced into the melt through the surface of the permeable section.

[0036] The porous or permeable section (16) functions as the contact surface between the blowing agent and the polymer melt. The change in the screw base's depth leads to a drop in pressure in this section (16). The compressed blowing agent, e.g. an expanding fluid, is fed through the bore (20) along the screw's longitudinal axis and one or more radial bores (22) for distributing the blowing agent, for example, through the permeable sintered metal surface. The design of this porous or permeable section (16) can take, for example, the form of a sleeve, cylinder, hollow cylinder or consist of at least one of the plugs or rings used in the screw's shaft.

[0037] The porous or permeable section (16) can be formed from sintered metal or other permeable material such as, for example, ceramic.

[0038] The bores (20, 22) are upstream of the feed hopper (5) and connected to an blowing agent feed device. This connection is effected by a sealing case (24) with a core and a screw-on cover that surrounds the plasticizing screw (3).

[0039] The sealing case (24) is installed between the plasticizing cylinder (2) and a drive mechanism (not depicted) for the plasticizing screw (3) and is secured against twisting. The sealing case (24) moves simultaneously with the axial movement of the plasticizing screw (3). The axial travel of the plasticizing screw (3) corresponds, for example, to three times the diameter of the plasticizing cylinder (2). The sealing case (24) is fitted with special rotary seals and is centered on the screw's shaft with slip rings. Mechanical clamping pieces prevent an axial displacement of the sealing case (24). Slip ring seals or rotary shaft seals can be used as the rotary seals (26). One or more radial bores (28) connect the pressure chamber of the blowing agent feed device (18) to the axial bore (20) along the longitudinal axis of the plasticizing screw (3).

[0040] When the blowing agent is distributed over the surface of the section (16), shearing elements (30) and mixing elements (32), both designed to function as pumps, distribute and disperse the polymer/blowing agent solution. Both the geometry and the order of the shearing elements (30) and mixing elements (32) may deviate from that depicted in the only figure.

[0041] An advantageous feature is that the blowing agent feed device (18) is supplied with the blowing agent from commercially available pressure cylinders. During each of the stages for dosing the polymeric material, an electrically, pneumatically, or hydraulically actuated valve (34) connects the blowing agent supply—if necessary throttled through a pressure-reducing valve—to the high-pressure sealing case (24). 

1) an injection molding machine for the injection molding of foamed molded parts of polymeric materials, especially thermoplastic materials, by introducing a physical blowing agent into the melt flow via a plasticizing screw (3) rotating and moving in axial direction in a plasticizing cylinder (2) characterized in that the plasticizing screw (3) upstream of a metering zone has a porous or permeable section (16) which is pressurized with the blowing agent supplied by an blowing agent feed device (18) in the screw shaft of the plasticizing screw (3) and which introduces the blowing agent through the whole surface of the melt: 2) A device according to claim 1, characterized in that the diameter of the plasticizing screw (3) is reduced in the area of the porous or permeable section (16) of the plasticizing screw. 3) A device according to one of claims 1 or 2, characterized in that during the dosing phase the blowing agent can be fed to the plasticizing screw (3) through a high-pressure sealing case (24) radially enclosing the plasticizing screw (3). 4) A device according to one of claims 1 to 3, characterized in that the length of the plasticizing cylinder (2) is twenty to twenty-five times that of the diameter. 5) A device according to one of claims 3 or 4, characterized in that the high-pressure sealing case (24) is supplied with the blowing agent from at least one pressure cylinder. 6) A device according to one of claims 3 to 5, characterized in that the high-pressure sealing case (24) moves, without rotating, in the axial direction simultaneously with the axial movement of the plasticizing screw (3). 7) A device according to one of claims 3 to 6, characterized in that rotary shaft seals (26) with pre-stressed springs seal the high-pressure sealing case against the rotating plasticizing screw (3). 8) A device according to one of claims 3 to 6, characterized in that slip ring seals seal the high-pressure sealing case (24) against the rotating plasticizing screw (3). 9) A device according to one of claims 3 to 8, characterized in that the porous or permeable section (16) forms a surface of a porous or permeable material in contact with the polymer melt in the plasticizing cylinder (2). 10) A device according to one of claims 3 to 9, characterized in that the porous or permeable section (16) consists of a sleeve, cylinder, hollow cylinder, or at least one plug or ring used in the screw shaft. 11) A device according to one of claims 1 to 10, characterized in that the porous or permeable section (16) is made of a porous or permeable material, for example sintered metal or a ceramic material. 12) A device according to one of claims 1 to 11, characterized in that the plasticizing screw (3) has along its longitudinal axis a bore (20) through which the blowing agent can be fed to the porous or permeable section (16). 13) A device according to one of claims 1 to 12, characterized in that the plasticizing screw (3) upstream of the porous or permeable section (16) has shearing elements (30) on the outer circumference of the plasticizing screw (3). 14) A device according to one of claims 1 to 13, characterized in that the plasticizing screw (3) upstream of the porous or permeable section (16) has mixing elements (32) on the outer circumference of the plasticizing screw (3). 15) An injection molding method that uses a physical blowing agent for manufacturing foamed molded parts by plasticizing a polymer introduced into the feed zone (6) of a plasticizing screw (3), by introducing an blowing agent into the flow of polymer melt through a flat, rotating gas injection area that also moves along the axial direction and that is formed by a porous or permeable section (16) of a plasticizing screw (3), and by injecting the homogenized polymer/blowing agent solution into a mould. 16) An injection molding method according to claim 15, characterized in that the polymer/blowing agent solution is homogenized by mixing and shearing elements (30, 32) that are affixed to the plasticizing screw (3) and that always exhibit the same effective lengths during the course of gas injection. 17) An injection molding method according to one of claims 15 or 16, characterized in that no dosing station is needed to feed the blowing agent directly from pressure cylinders via the sealing case (15) of the plasticizing screw (7). 18) An injection molding method according to one of claims 15 to 17, characterized in that the blowing agent is injected during the dosing phase. 19) An injection molding method according to one of claims 15 to 18, characterized in that the shearing elements (30) on the plasticizing screw (7) upstream of the gas injection area serve to improve the distribution of the blowing agent in the melt further on in the plasticizing unit, and to shorten the diffusion paths. 20) An injection molding method according to one of claims 15 to 19, characterized in that mixing elements (32) on the plasticizing screw (3) homogenize the heat and the substance of the polymer/blowing agent solution. 